1. Field of the Invention
The present invention relates to a surface acoustic wave resonator in which reflectors are disposed adjacent to both ends in the surface acoustic wave propagation direction, of an IDT electrode, and a ladder-type filter including the surface acoustic wave resonator. In particular, the present invention relates to a surface acoustic wave resonator in which cross widths of an IDT electrode are weighted, and a ladder-type filter including the surface acoustic wave resonator.
2. Description of the Related Art
Surface acoustic wave resonators have been widely used in communication devices, such as cellular phones, so as to define resonators or filters.
For example, Japanese Unexamined Patent Application Publication No. 2000-286663 discloses a surface acoustic wave resonator 1001 shown in FIG. 14. The surface acoustic wave resonator 1001 includes an electrode structure provided on a piezoelectric substrate. The surface acoustic wave resonator 1001 is a surface acoustic wave resonator using Love waves having an electromechanical coupling coefficient k2 greater than that of Rayleigh waves.
In the surface acoustic wave resonator 1001, an IDT electrode 1002 is provided on the piezoelectric substrate. The IDT electrode 1002 includes a bus bar 1003 and a bus bar 1004 that is opposed to the bus bar 1003. The bus bar 1003 includes a bus bar portion 1003a extending in an inclined direction forming an angle of θ with the surface acoustic wave propagation direction and a bus bar portion 1003b extending in an inclined direction forming an angle of −θ with the surface acoustic wave propagation direction and connected to the bus bar portion 1003a. 
Similarly, a second bus bar 1004 includes a bus bar portion 1004a extending in an inclined direction forming an angle of −θ with the surface acoustic wave propagation direction and a bus bar portion 1004b connected to the bus bar portion 1004a and extending in an inclined direction forming an angle of θ with the surface acoustic wave propagation direction.
The bus bar portions 1003a and 1003b and bus bar portions 1004a and 1004b form a substantial rhombus.
Multiple electrode fingers 1005 extend from the bus bar portions 1003a and 1003b toward the bus bar portions 1004a and 1004b. Dummy electrodes 1006 are arranged so as to oppose the tips of the electrode fingers 1005 with gaps therebetween. Ends of the dummy electrodes 1006 are connected to the second bus bar 1004 and the other ends thereof are opposed to the electrode fingers 1005 with the above-mentioned gaps therebetween.
Similarly, multiple electrode fingers 1007 including ends connected to the second bus bar 1004 and the other ends extending toward the first bus bar 1003 are provided. Dummy electrodes 1008 are disposed so as to oppose the tips of the electrode fingers 1007 with gaps therebetween in the length direction of the electrode fingers. Ends of the dummy electrodes 1008 are connected to the first bus bar 1003 and the other ends thereof are opposed to the electrode fingers 1007 with the above-mentioned gaps therebetween.
The electrode fingers 1005 and the electrode fingers 1007 are alternately disposed in the surface acoustic wave propagation direction. In addition, in the above-mentioned IDT electrode 1002, the cross widths are weighted. Based on this cross-width weighting, the cross width located at the center in the surface acoustic wave propagation direction is the greatest and cross widths located outward in the surface acoustic wave propagation direction are reduced therefrom.
In the surface acoustic wave resonator 1001, the cross width of the above-mentioned smallest cross width portion is zero. Areas in which only the dummy electrodes 1006 and 1008 exist are provided at the ends in the surface acoustic wave propagation direction.
A feature of the surface acoustic wave resonator 1001 is that the cross widths are weighted as described above and an envelope A obtained by the above-mentioned cross width weighting is in parallel with the inner edges of the bus bar portions 1003a, 1003b, 1004a, and 1004b. In other words, the inner edges of the bus bar portions 1003a to 1004b are disposed in parallel with the envelope. That is, the inner edges of the bus bar portions 1003a to 1004b are inclined at an angle of θ or −θ with the surface acoustic wave propagation direction. For this reason, resonance caused by a harmonic higher-order mode does not readily occur. This reduces spurious waves. Particularly, in Japanese Unexamined Patent Application Publication No. 2000-286663, portions between the above-mentioned envelope and the inner edges of the bus bars extending in parallel with the envelope function as reflectors. For this reason, for example, as shown by a straight line L in FIG. 15, an excited surface acoustic wave crosses, for example, five electrode fingers before reaching the inner edge of the bus bar portion 1003b. Since these five electrode fingers function as reflectors, spurious waves can be effectively restrained, which enables downsizing.
Surface acoustic wave resonators are used to define oscillation circuits, filters, and other suitable devices. To define a filter circuit, a plurality of surface acoustic wave resonators are typically connected. For example, in a ladder-type filter including a plurality of surface acoustic wave resonators, at least one surface acoustic wave resonator is connected to a series arm and at least one surface acoustic wave resonator is connected to a parallel arm. For a ladder-type filter, the attenuation is not sufficiently increased in a band higher than the passband if the impedance of the series arm resonator at the anti-resonance frequency is not sufficiently increased.
For a parallel arm resonator, insertion loss may increase in the passband if the impedance thereof at the anti-resonance frequency is not sufficiently increased.
However, for the surface acoustic wave resonator described in Japanese Unexamined Patent Application Publication No. 2000-286663, the impedance thereof at the anti-resonance frequency may not be sufficiently increased. There is also a problem in that return loss increases in a frequency band higher than the anti-resonance frequency, for example, at a frequency that is approximately 1.003 times the anti-resonance frequency. For this reason, insertion loss may increase in a band higher than the passband of a ladder-type filter using the surface acoustic wave resonator as a parallel arm resonator.